1. Field of the Invention
The present invention relates generally to programmable logic controllers and, more specifically, to a method of part flow model for a programmable logic controller logical verification system for manufacturing a motor vehicle.
2. Description of the Related Art
It is known that programmable logic controller code is written by controls engineers after assembly tooling designs are completed and a manufacturing process has been defined. The creation of the programmable logic controller code is mostly a manual programming task with any automation of the code generation limited to “cutting and pasting” previously written blocks of code that were applied to similar manufacturing tools. Once the programmable logic controller code is written, it is used by a programmable logic controller to build subsequent hard tools used in the manufacture of parts for motor vehicles. The programmable logic controller code is not validated (debugged) until the hard tools are built and tried. A significant portion of this tool tryout process is associated with the debugging of the programmable logic controller code at levels of detail from a tool by tool level, to a workcell level, and finally at a tooling line level.
It is also known that a manufacturing line is typically made of three to twenty linked workcells. Each workcell consists of a fixture to position product (sheet metal) and associated automation (robots) that process the product (welding). The workcell typically consists of a fixture/tool surrounded by three or four robots. The product is then transferred to the next workcell in the manufacturing line for further processing, until it exits the manufacturing line.
It is further known that the workcells for a manufacturing line can be modeled before the manufacturing line is implemented. The modeling techniques, such as Robcad from Tecnomatix and Igrip from Deneb, for the manufacturing process are limited in scope to a workcell level, due to how these type of technologies represent and manipulate three dimensional data and tool motions. It is still further known that there are two PLC simulation systems commercially available, one from SST called PICS and the other from CAPE Software called VPLink. However, neither simulation system has an explicit part flow model for discrete part manufacturing. It is further known that part flow model simulation software exists known as “Discrete Event Simulation”. Although Discrete Event Simulation software uses an explicit part flow model, the parts are active and engage resources as they move through the system. The software also makes use of an event clock that is incompatible with the needs of the PLC emulation for continuous time.
In manufacturing plant floor operations, programmable logic controllers (PLCs) execute PLC code through sensing where parts are located within the tooling by using sensors/switches. The representations of parts and part flow are unique and necessary within a Virtual PLC (VPLC) to accurately portray the logical condition of the manufacturing process to the inputs of a PLC. Without part movement in the VPLC that can be “sensed”, the emulated PLC would be unable to exercise its code for even its basic function of automatic sequencing.
Therefore, it is desirable to provide a method for application of a part flow model as part of a programmable logic controller logical verification system. It is also desirable to provide a method for logical modeling and simulation of parts that hold information specific to manufacturing processes that the parts have been routed through. It is further desirable to provide a method of part flow as a component of the VPLC to allow additional uses beyond verifying PLC logic such as routing and quality testing. Therefore, there is a need in the art to provide a method that meets these desires.